The delivery of polynucleotide and other membrane impermeable compounds into living cells is highly restricted by the complex membrane systems of the cell. Drugs used in antisense and gene therapies are relatively large hydrophilic polymers and are frequently highly negatively charged as well. Both of these physical characteristics preclude their direct diffusion across the cell membrane. For this reason, the major barrier to polynucleotide delivery is the delivery of the polynucleotide to the cellular interior. Numerous transfection reagents have been developed to deliver polynucleotides to cells in vitro. However, in vivo delivery of polynucleotides is complicated by toxicity, serum interactions, and poor targeting of transfection reagents that are effective in vitro. Transfection reagents that work well in vitro, cationic polymers and lipids, typically destabilize cell membranes and form large particles. The cationic charge of transfection reagent facilitates nucleic acid binding as well as cell binding. Destabilization of membranes facilitates delivery of the membrane impermeable polynucleotide across a cell membrane. These properties render transfection reagents ineffective or toxic in vivo. Cationic charge results in interaction with serum components, which causes destabilization of the polynucleotide-transfection reagent interaction and poor bioavailability and targeting. Cationic charge may also lead to in vivo toxicity. Membrane activity of transfection reagent, which can be effective in vitro, often leads to toxicity in vivo.
For in vivo delivery, a transfection complex (transfection reagent in association with the nucleic acid to be delivered) should be small, less than 100 nm in diameter, and preferably less than 50 nm. Even smaller complexes, less that 20 nm or less than 10 nm would be more useful yet. Transfection complexes larger than 100 nm have very little access to cells other than blood vessel cells in vivo. In vitro complexes are also positively charged. This positive charge is necessary for attachment of the complex to the cell and for membrane fusion, destabilization or disruption. Cationic charge on in vivo transfection complexes leads to adverse serum interactions and therefore poor bioavailability. Near neutral or negatively charged complexes would have better in vivo distribution and targeting capabilities. However, in vitro transfection complexes associate with nucleic acid via charge-charge (electrostatic) interactions. Negatively charged polymers and lipids do not interact with negatively charged nucleic acids. Further, these electrostatic complexes tend to aggregate or fall apart when exposed to physiological salt concentrations or serum components. Finally, transfection complexes that are effective in vitro are often toxic in vivo. Polymers and lipids used for transfection disrupt or destabilize cell membranes. Balancing this activity with nucleic acid delivery is more easily attained in vitro than in vivo.
While several groups have made incremental improvements towards improving gene delivery to cells in vivo, there remains a need for a formulation that effectively delivers a polynucleotide together with a delivery agent to a target cell without the toxicity normally associated with in vivo administration of transfection reagents. The present invention provides compositions and methods for the delivery and release of a polynucleotide to a cell using biologically labile conjugate delivery systems.